1. Field of the Invention
The present invention relates to a noncircular shape machining apparatus such as an NC lathe or an NC grinder that turns, and cuts or grinds a rotating work to produce a manufactured article having a noncircular cross section.
2. Description of the Related Art
As an example of a conventional noncircular shape machining apparatus, the configuration described in JP-A-5-173619 that has the function of correcting machining error is known. Here, operation of a conventional noncircular shape machining apparatus will be described as an example of the machine configuration described in JP-A-5-173619. FIG. 7 is a block diagram of the apparatus, FIG. 8 is a machine configuration diagram (general front diagram) of this conventional noncircular shape machining apparatus.
As shown in FIG. 8, a workpiece 1 is rotated at a certain rotational speed by a spindle motor 3, and the rotational angle is detected by a spindle encoder 4. A tool 7 is attached to a cutting tool carriage 8 that is rectilinearly driven by an X-axis motor 10 so as to reciprocally move in the radial direction (X-axis direction) of the workpiece 1 in accompaniment with the rotation of the X-axis motor 10, and the moving distance is detected by an X-axis linear scale 12. Additionally, the cutting tool carriage 8 is moved forward and backward by the driving of the X-axis motor 10, which is synchronized by a controller with the rotation of the workpiece 1 on the basis of the detected value from the spindle encoder 4, and the workpiece 1 is turned and cut by the tool 7 attached to the cutting tool carriage 8. Further, in order to cause the tool 7 to move in the longitudinal direction of the workpiece 1 (Z-axis direction, direction perpendicular to the page), an X-axis movable section 9 including the cutting tool carriage 8 and a middle carriage is structured such that it can move in an orthogonal direction over a bed 14 by combined use of a Z-axis motor 13 and a saddle 11, which is immovable with respect to the X axis.
FIG. 7 is a functional block diagram of the controller that generates moving commands of the tool 7 synchronously with the angle detected by the spindle encoder 4, and the control contents will be described by reference to this diagram. First, the workpiece 1 is caused to rotate at a desired rotational speed by the spindle motor 3. The spindle encoder 4 detects the rotational angle of the workpiece 1 and transmits a two-phase sinusoidal signal to a spindle encoder interface 24, and the spindle encoder interface 24 outputs the rotational angle θ of the workpiece 1. A target position f(θ) of the tool 7 or a value approximating the target position f(θ) is stored beforehand as a command position c(θ) in a command position data memory 21, and a command position data reading unit 22 reads the command position c(θ) corresponding to the rotation angle θ of the workpiece 1 from the command position data memory 21 and controls an X-axis servo system 23. Additionally, a detected position data writing unit 26 causes the position a(θ) of the tool 7 detected by the X-axis linear scale 12 to be stored in a detected position data memory 25 with regard to one rotation of the workpiece 1; that is, a range where θ is from 0° to 360°. The target position f(θ) of the tool 7 is stored beforehand in a target position data memory 27, and a command position data correcting unit 20 causes storage, in the command position data memory 21, of a corrected command position cc(θ) in which the command position c(θ) has been corrected by expression 1 below on the basis of the difference between the target position f(θ) read from the target position data memory 27 and the detected position a(θ) read from the detected position data memory 25.cc(θ)=c(θ)+(f(θ+Δθ)−a(θ+Δθ))  expression 1
Additionally, the command position data reading unit 22 reads, from the command position data memory 21, the corrected command position cc(θ) corresponding to the rotational angle θ of the workpiece 1 that has been read from the spindle encoder interface 24 and controls the X-axis servo system 23. Additionally, the detected position data writing unit 26 causes the position a(θ) detected by the X-axis linear scale 12 to be stored in the detected position data memory 25 with regard to one rotation of the workpiece 1; that is, a range where θ is from 0° to 360°. Additionally, the command position data correcting unit 20 determines whether or not the deviation between the detected position a(θ) that has been read from the detected position data memory 25 and the target position f(θ) that has been read from the target position data memory 27 is equal to or greater than a certain value, and when the deviation is equal to or greater than the certain value, the command position data correcting unit 20 ends all processing. On the other hand, when the deviation is not equal to or greater than the certain value, the command position data correcting unit 20 substitutes the command position c(θ) with the corrected command position cc(θ) and repeats the aforementioned operation.
Incidentally, the aforementioned conventional noncircular shape machining apparatus implements control under the assumption that the output of the X-axis linear scale 12 in FIG. 8 is equal to the relative moving distance between the workpiece 1 and the tool 7. However, when the tool 7 is reciprocally moved at high acceleration on the X axis, or the weight of the tool 7 and the X-axis movable part 9 is heavy, the X-axis movable section 9 receives an accelerating/decelerating reaction force, and the saddle 11 that is the immovable part on the X axis periodically oscillates in the opposite direction. In terms of the entire machine, the center-of-gravity position of the machine does not move by one rotation of the spindle; hence, wobbling of the saddle 11 resulting from X axis reaction and wobbling where the workpiece 1 periodically oscillates via the bed 14 assume substantially opposite phases, and sometimes the change in the relative distance between the two reaches several microns. For this reason, there has arisen the problem that, even when the output value of the X-axis linear scale 12 is controlled to match the target position, the workpiece cannot be machined as intended to its desired outer shape, because of oscillation of the entire machine resulting from the acceleration/decelerating reaction force of the X-axis movable section 9.